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Geologic Time and Climate Change Science

Posted on 18 February 2013 by rockytom

The concept of geologic time is difficult to convey to introductory science students at any level.  In fact, it is a difficult concept for anyone.  The use of an analogy is probably the most effective way to convey this concept, for example, if a person extended his or her arms at their sides horizontally, the time represented by human history on Earth can be represented by passing a nail file one time over the index finger nail.  The time represented by the total length of the outstretched arms would represent the length of geologic time.  In the new textbook by Farmer and Cook (“Climate Change Science: A Modern Synthesis, Volume 1, The Physical Climate”), geologic time and events are a recurrent theme throughout the text.  The second volume in the series will be subtitled “Earth’s Climate History” and the emphasis will be on the most recent climate changes because those are what we know the most about.

Given that Earth is around 4.5 billion years old and the first 4 billion years or so is little known compared to the last .5 billion years, there is spotty evidence for climate change in the geologic record prior to the Cambrian Period (beginning about 540 million years ago).  Near the beginning of the Cambrian (slightly before 540 million years ago) rocks of Earth’s crust began to yield fossils of organisms that once lived in certain environments compared to current ones.  Many of these organisms, protists (single celled organisms), plants, and animals, lived in shallow marine waters and paleogeographers are able thereby to map ancient shorelines and the distribution of ancient seas.

Previous SkS Posts on Geologic Time

In an article reposted by John Hartz on this site on 05 March 2012 (a reprint of a news release by the National Science Foundation on 01 March 2012), ocean acidification was discussed as progressing faster at present than at any time in the past 300 million years and the last time it was even close to the present was during the Paleocene-Eocene Thermal Maximum (PETM), beginning about 56 million years ago.  During the PETM, in about 5,000 years atmospheric carbon dioxide doubled to 1,800 parts per million (ppm), and average global temperatures rose by about 6 degrees Celsius, according to the article.

A representation of the geologic time scale furnished by John Mason is given below in English and Welsh.


The majority of climate change science papers only go back in time to the start of the Pleistocene, commonly known as the “Ice Age” about 2.5 million years ago.  By studying the redundant glacial advances and retreats beginning around 650,000 years ago it is thought that we might better understand the climates of the present and future. 

Earth’s global climate has changed rather abruptly in the past; in the case of ice advances and retreats, the demise of the dinosaurs, the Permo-Triassic extinction, and the PETM.  It is certainly possible for Earth’s climate to again change abruptly.

Some have postulated that Earth was once a “snowball” and there is some geologic evidence for this.  There may have been several time intervals during which the entire Earth was covered in ice but these were far back in Earth history (during the Neoproterozoic) and the evidence is not entirely convincing.  Even if true, a snowball Earth does not look like it will be in the planet’s future.

In Volume 2 of the Farmer and Cook textbook series, “Earth’s Climate History,” we will start with a survey of present day climate and continue backward in time to the Huronian glaciation (during the Paleoproterozoic from 2.4 to 2.1 billion years ago).  Of course, most of the geologic evidence for climate change is from the Pleistocene.  As one travels back in time, the picture of Earth’s climate becomes less and less clear.

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Comments 1 to 13:

  1. One of the most creative takes on this subject is Mark Twain's short satirical essay "Was the Earth Made for Man?" Twain wrote this piece around 1900 when Darwin's theory of evolution had come under attack from scientists who, unaware of the processes of atomic decay that keep the earth's core nice and toasty warm, argued that our planet could not possibly be old enough to allow for all the time Darwin's theory seemed to require. One of the chief scientists in this debate was Lord Kelvin. Along with others, Kelvin believed thermodynamic realities limited the earth's age to 100 million years or less and he spent a lot of his intellectual energy in his later years making this point.

    In his essay, Twain engages in the debate and uses the Eiffel Tower instead of a human's outstretched arms to indicate the span of time involved, and likens our time on the planet to "the skin of paint on the pinnacle knob at its summit." He has a great time pointing out that the poor deluded oyster, who showed up 19 million years into the scheme, might well be excused for thinking that the earth was created for him, but goes on to explain that the oyster was just one of many things created for man as part of God's great plan. It really is quite good.

    The essay was suppressed more or less willingly by Twain during his lifetime and was not published until 1938, when it appeared in Letters from the Earth, a collection of Twain's most irreverent writings. Incidentally, the letters referenced in the title are purported to have been written by Satan about what he has learned about Man, God's great creation, to his best buddies Michael and Gabriel back in heaven and include a letter about how the Ark had to turn back to recover a housefly in order to preserve, not exactly for humanity's benefit, typhoid germs.

    Anyway, tracking down a copy likely involves purchasing this book. Of course, the book is well worth the price, as it also contains things like the delightfully witty "Extracts from Eve's Autobiography," which is found in a series of similar spoofs under the heading "Papers of the Adam Family." In Eve's autobiography, we learn that Adam was the first scientist, and his first important discovery was that water flowed downhill. The book is thankfully readily available in a paperback reprint published by HarperPerennial.


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  2. Don, I enjoyed the analogy.  Putting it in terms of the figure above:

    The last section (10) is the duration since the extinction of the dinosaurs.

    The black line (1 pixel wide) represents the time since humans diversified from our common ancestors with Chimpanzees.

    Homo sapiens has been around for 4% of the final pixel.

    Civilization has been around for 0.2% of the final pixel. 

    And the time since the industrial revolution represents the last 0.003% of the final pixel.

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  3. Twain essay here

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  4. Glad to provide it, Tom. Stephen Jay Gould wrote a number of natural history essays that touch on deep time, sometimes directly, sometimes less so. In one that I recall, he pointed out that early on in the evolution of vertebrates there was a great deal of "experimentation" on just how many digits a limb should have. He cited multiple examples of animals with more than the usual five to make the point that evolution spun out many different options at various points in deep time, and I believe he also pointed out that the fossil record is so fragmentary that we really have no idea just how rich life was way back when.

    In the same way we have a hard time reconstructing climate conditions from way back then, we also don't have a good sense of what complete ecosystems looked like even a few thousand years ago. We only need consider some very recent extinctions or near extinctions to grasp that modern humans inhabit a landscape that, despite our perceptions to the contrary, is dramatically different from what it was only a couple hundred years ago. The two things I'm thinking of are the American chestnut and the Passenger Pigeon. Both were ubiquitous in the eastern third of the US and their absence has altered the physical landscape we inhabit in ways we will probably never fully understand. I suspect global warming, even if we prevent its most catastrophic impacts, will, two hundred years hence, have effected staggering changes to the places we call home.

    This will be a great cultural loss, amongst other things. When I travel in England, I often have the sense that the landscape I'm seeing, with its fields, hedgerows, and woodlots and their accompanying array of species, is still relatively similar to what Shakespeare or William the Conqueror would have seen. I doubt people in two hundred years will be able to maintain that same connection. In other words, global warming, like indiscriminate hunting or the introduction of a new virulent disease, acts like an accelerant on the natural processes of change that go on all the time.

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  5. Good work, Chris!

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  6. The excellent "fingernail" analogy, which rockytom uses was first used (I think) by John McPhee in his wonderful book  Basin and Range, which I would recommend to everyone. Stephen Jay Gould ( no slouch himself when it came to writing on science) later used the metaphor in his book Time's Arrow, Time's Cycle. His version, as quoted on the Wikipedia article on Deep Time is as follows:

    Consider the Earth's history as the old measure of the English yard, the distance from the King's nose to the tip of his outstretched hand. One stroke of a nail file on his middle finger erases human history.

    I touched on the importance of understanding deep time and basic geological principles in my SkS article on my own struggle to come to terms with climate change.

    More recently,  Andrew Glikson has shown here how current anthropogenic processes are proceeding at a rate far faster than past natural processes.

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    Moderator Response: [rockytom]You are correct, Andy. It was McPhee's use of the analogy that I was thinking of when I wrote the above post. Stephen Jay Gould was one of my favorite people and he also used the analogy. I guess I don't know who first used it. I met Gould in the mid 1970s and was saddened by his untimely death.
  7. Here is my attempt to illustrate the vast evolutionary timeline, not the geological timeline. (I know it's know outdated).

    You can find a desciption of an imaginary movie, taking one year to view, depicting evolution on earth.

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  8. the first 4 billion years or so is little known compared to the last .5 billion years

    I though it was typo until I spotted the dot when I pasted it here :). it'd better be "last 500 million".

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  9. I question the choice of graphing geological time in a closed-circle fashion around the calendar year, as if the Earth's evolution is coming to an end, presumably with us humans writing the final chapter at the end of the Quaternary, as the Holocene becomes the Anthropocene and then, boom, it's Dec. 31st. If we do in fact push the Earth System into a "state shift" (, it will not be the end of physical evolution on the Earth, nor is it likely that Life will come to an end--it will just be knocked back for some millions of years, as with other major extinction events, tho it will be too bad for us and our our own evolutionary cohorts.

    Also--hope this is not too much "off topic," since i'm a new arrival here from the Climate Ethics website--what thoughts do folks here have about waking our species up to what it's doing and bringing about a change in the whole paradigm that dictates how we organize our human activities? I can envision a kind of discontinuity or "fold bifurcation" in the evolution of how we humans think coming soon too, but I believe it's something we can speed up if enough of us focus our collective intentionality on making change.

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    Moderator Response: [rockytom] One can think of the top of the circle as being open to the future. I guess you can show it without drawing the vertical line. The line at the top was not intended to be the end of the world as we know it and there are even more elaborate circles and spirals drawn to represent geologic time.
  10. Slightly off topic, something (amongst a long list) that puzzles me.  Are there any indications why sea level during the Eemian interglacial was about 5m above present sea level.  What led to the extra melting of Greenland and, as now seems likely, the collapse of the West Antarctic Ice Sheet.  How come we didn't get to these sea levels by, say, the Roman times.

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  11. Firstly, it takes 1000s of years to melt an icesheet. Milankovich-cycle driven climate change is very slow compared to modern climate change so the warming persisted long enough for it to happen. Maintain current temperatures and we will get there.

    Second, dont assume that Roman optimum was global. If you look at say Schaefer et al 2009 you will see it was cold down here.

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  12. Re 12

    In NH, Holocene interglacial maximum received LESS summer insolation than Eemian interglacial maximum. New paper in Nature (NEEM (Dahl-Jensen et a) 2013) shows Greenland survived this, allbeit shedding half its mass. This means that because the Eemian sea level data are hard to ignore, the WAIS likely disintegrated (at least partially).

    Not sure what the future has in store for WAIS, mainly because no one can know, but the science is progressing as we speak (and it is very interesting...)

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  13. The analogies mentioned may offer a sense for the large-scale but at expense of the small scale which disappears into imperception. To deal with this, i highlight the smaller scale with a length analogy where a year corresponds to 1cm... a millenium would be 1m, a million years 1km, and a billion years 1000km.

    So, our 20cm of polluting dominance tail our few meters of civilisations and come at the end of our species' few kilometres of life that stretch onto a continental-scale backdrop.

    I like it for easily differentiating orders of magnitude and reflecting the evolution time-scales and i'm pretty pleased that my primary-school nieces like it.

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